The present invention relates to a rubber composition or, more particularly, a rubber composition curable by hot air vulcanization under atmospheric pressure based on an ethylene-propylene copolymeric rubber.
As is well known, rubber compositions formulated with an ethylene-propylene copolymeric rubbery elastomer are susceptible to the vulcanization both with an organic peroxide and with sulfur or an organic sulfur compound as the vulcanizing agent. They can be shaped into various forms of shaped rubber articles by the techniques of compression molding in a metal mold, extrusion molding with an extruder machine and other conventional molding methods. Shaped and cured articles of ethylene-propylene copolymeric rubbers have found wide applications in a variety of fields in recent years such as the parts of automobiles, insulating covering of electric wires, gaskets in buildings and the like including general-purpose rubber sheets by virtue of their excellent heat resistance, anti-cold resistance, electric characteristics and the like.
The vulcanization of these rubbers is characteristically performed always under pressurized condition as is the case in the above mentioned vulcanization processes. Needless to say about the compression molding, the vulcanization in the extrusion molding is also performed with a pressurized steam in general in the molding process for manufacturing electric wires, tubes and any other extruded products having irregular cross sections.
One of the problems in the vulcanization with pressurized steam is that the temperature of the steam available in most of the rubber processing factories is 200.degree. C. at the highest or, in general, in the range from 150.degree. to 170.degree. C. Even though the pressure of the steam in the above temperature range is sufficient for the vulcanization of the rubbers, the temperature per se is not always sufficently high to give a velocity of vulcanization desired from the standpoint of productivity. That is, the velocity of steam vulcanization is unduly low in comparison with the velocity of extrusion from the die of an extruder machine so that, when the extrusion and steam vulcanization must be continuously and successively carried out as in the manufacturing of rubber insulated electric wires, sufficient productivity is obtained only in facilities equipped with a very low vulcanization tube constructed with large expenses since otherwise the velocity of extrusion cannot be so high and the overall productivity is greatly reduced with the velocity of the vulcanization as the rate determining step.
Moreover, the steps of extrusion and vulcanization sometimes cannot be performed continuously and successively depending on the forms of the extruded materials so that the steps must be practiced separately and, needless to say, the productivity is far from satisfactory.
On the other hand, several rubbery elastomers are known to be vulcanizable by heating in hot air under atmospheric pressure. For example, silicone rubbers formulated with certain kinds of vulcanizing agents are vulcanized when heated at a high temperature of, usually, 250.degree. C. or higher for an extremely short period of time under atmospheric pressure by passing through a zone of hot air atmosphere following extrusion from the extruder die continuously. If desired, the temperature of the hot air zone can be further increased to 350.degree. to 400.degree. C. without particular difficulties so as that the time for the vulcanization is further shortened and, as a consequence, the velocity of extrusion is further increased contributing to a great improvement of the productivity as well as a decrease of the costs for the vulcanization facilities to an incomparably lower extent than for the facilities for the steam vulcanization.
There has recently been disclosed that the temperature of vulcanization of the ethylene-propylene copolymeric rubbers can be decreased when the vulcanizing agent is cumene hydroperoxide combined with a cobalt or chromium compound (see Nippon Gomu Kyokai Shi, volume 51 (1978), No. 5, page 274) but, even in this method, the vulcanization must be carried out under pressure with high temperature steam and hot air vulcanization under atmospheric pressure cannot be undertaken as in the vulcanization of silicone rubbers. In addition, the above mentioned vulcanizing agent is susceptible to ionic decomposition when the rubber composition is formulated with a strongly acidic filler such as clay or silica and the radical reaction necessary for the crosslinking reaction is sometimes hindered so that the desired vulcanization of the rubber is retarded all the more.